Field of the Invention
The present invention relates to a throttle valve, and in particular to a throttle valve that is protected from particle pollution.
Description of the Related Art
The process of depositing layers on a semiconductor Wafer (or substrate) usually involves heating the substrate and holding it a short distance from the source of a stream of deposition (or process) gas flowing towards the substrate. The flowing gas reacts and deposits a layer on the heated substrate.
At some point in the vacuum line between the chamber and the vacuum pump, a vacuum shut-off or throttle valve is typically provided which, when closed, acts as a limit of the process chamber containment. Volatile contaminants which are maintained in their vapor state under the high temperatures of the active processing chamber can and do undesirably condense on the walls of the cool vacuum piping some distance from the processing area of the processing chamber, but still within the containment limit of the processing chamber defined by the vacuum valve. During no-flow conditions, these contaminants can migrate back into the processing part of the chamber to undesirably contribute to its contamination. In an attempt to avoid this problem, the inside surfaces within the containment limit of the processing chamber are periodically cleaned by etching (and/or plasma cleaning) these surfaces with a cleaning gas to remove the dielectric material deposited by the deposition gas. Optimization involves extending the time between cleanings as much as possible without introducing defects. Cleaning interferes with the normal production process.
However, pollution in the form of particulate matter is usually deposited on the inner elements of the throttle valve, and this decreases the throttle valve's ability to control air flow. Throttle valves are usually controlled by a stepper motor. When the particle pollution is deposited on the inner elements of the throttle valve, the stepper motor repeatedly modifies the degree of opening of the throttle valve, and the pressure within the processing chamber becomes unstable, decreasing the lifetime of the throttle valve.